Nickel-based heat-resistant superalloy

ABSTRACT

Disclosed herein is a nickel-based heat-resistant superalloy produced by a casting and forging method, the nickel-based heat-resistant superalloy comprising 2.0 mass % or more but 25 mass % or less of chromium, 0.2 mass % or more but 7.0 mass % or less of aluminum, 19.5 mass % or more but 55.0 mass % or less of cobalt, [0.17×(mass % of cobalt content−23)+3] mass % or more but [0.17×(mass % of cobalt content−20)+7] mass % or less and 5.1 mass % or more of titanium, and the balance being nickel and inevitable impurities, and being subjected to solution heat treatment at 93% or more but less than 100% of a γ′ solvus temperature.

TECHNICAL FIELD

The present invention relates to a nickel-based heat-resistantsuperalloy used for heat-resistant members of aircraft engines,power-generating gas turbines, etc., especially for turbine disks orturbine blades.

BACKGROUND ART

For example, turbine disks, which are heat-resistant members of aircraftengines, power-generating gas turbines, etc., are rotary members thatsupport turbine blades, and are subjected to much higher stress thanturbine rotor blades. Therefore, turbine disks require a materialexcellent in mechanical characteristics, such as creep strength ortensile strength in a high-temperature and high-stress region andlow-cycle fatigue characteristics, and forgeability. On the other hand,in order to improve fuel efficiency or performance, an increase inengine gas temperature and a reduction in the weight of turbine disksare required, and therefore the material is required to have higher heatresistance and higher strength.

In general, nickel-based forged alloys are used for turbine disks. Forexample, Inconel 718 (which is a registered trademark of TheInternational Nickel Company, Inc.) using a γ″ (gamma double prime)phase as a strengthening phase and Waspaloy (which is a registeredtrademark of United Technoligies, Inc.) using, as a strengthening phase,about 25 vol % of a precipitated γ′ (gamma prime) phase stabler than aγ″ phase are frequently used. Further, Udimet 720 (which is a registeredtrademark of Special Metals, Inc.) has been introduced since 1986 fromthe viewpoint of dealing with higher temperatures. Udimet 720 has about45 vol % of a precipitated γ′ phase and tungsten added forsolid-solution strengthening of a γ phase, and is therefore excellent inheat-resistant characteristics.

On the other hand, the structural stability of Udimet 720 is not alwayssufficient, and a harmful TCP (Topologically close packed) phase isformed during use. Therefore, Udimit 720Li (U720Li/U720LI) has beendeveloped by making improvements, such as a reduction in the amount ofchromium, to Udimet 720. However, the formation of a TCP phase stilloccurs also in improved Udimit 720Li, and therefore the use of Udimit720Li for a long time or at high temperature is limited.

Powder metallurgical alloys typified by AF115, N18, and Rene88DT aresometimes used for high-pressure turbine disks required to have highstrength. The powder metallurgical alloys have a merit that homogeneousdisks having no segregation can be obtained in spite of the fact thatmany strengthening elements are contained. On the other hand, the powdermetallurgical alloys have a problem that their production process needsto be highly controlled, e.g., vacuum melting needs to be performed at ahigh cleaning level or a proper mesh size needs to be selected forpowder classification, to suppress the mixing of inclusions andtherefore their production cost is significantly increased.

In addition, many proposals have been made to improve the chemicalcompositions of conventional nickel-based heat-resistant superalloys.All of them contain cobalt, chromium, molybdenum or molybdenum andtungsten, aluminum, and titanium as their major constituent elements,and typical ones contain one or both of niobium and tantalum as theiressential constituent element(s). The presence of niobium and/ortantalum is suitable for the above-described powder metallurgy, but is afactor making casting and forging difficult.

Titanium is added for its function of strengthening a γ′ phase andimproving tensile strength or crack propagation resistance. However, theamount of titanium added is limited to up to about 5 mass %, becauseexcess addition of only titanium results in an increase in γ′ solvustemperature and formation of a harmful phase, which makes it difficultto obtain a sound γ/γ′ two-phase structure.

Under the circumstances, the present inventors have made a study ofoptimization of the chemical composition of a nickel-basedheat-resistant superalloy and have found that a harmful TCP phase can besuppressed by actively adding cobalt in an amount of up to 55 mass %.Further, the present inventors have found that a γ/γ′ two-phasestructure can be stabilized by increasing both a cobalt content and atitanium content so that cobalt and titanium are contained in apredetermined ratio. Based on these findings, the present inventors haveproposed a nickel-based heat-resistant superalloy that can withstandhigher temperatures for a long time than conventional alloys and thathas excellent workability (Patent Literature 1).

Further, some proposals focused on the microstructure of a nickel-basedheat-resistant alloy have been made to improve the performance of thenickel-based heat-resistant superalloy (Patent Literatures 2, 3, and 4).

In a nickel-based heat-resistant superalloy produced by powdermetallurgy, crystal grains are less likely to become too large evenafter solution heat treatment performed in a temperature regionexceeding a γ′ solvus temperature (at a supersolvus temperature), andtherefore crystal grain size and grain size distribution are generallycontrolled by performing aging heat treatment after solution heattreatment performed in a temperature region exceeding a solvustemperature (e.g., Patent Literature 7). However, while crystal grainsare less likely to become too large, it is often the case that thecontrol of crystal grains is poor. Therefore, in order to avoid harmfulgrowth of crystal grains during solution heat treatment performed in atemperature region exceeding a solvus temperature, the importance ofstrain rate control during forging has also been proposed (e.g., PatentLiteratures 5 and 6). Further, in order to promote proper growth ofcrystal grains, a method has also been proposed in which a nickel-basedheat-resistant alloy having a high carbon content is forged at a highlocal strain rate (Patent Literature 8).

However, the alloys described in the above Patent Literatures are powderalloys whose production process is complicated and production cost ishigh. The powder alloys vary in optimum microstructure according totheir chemical composition, and are therefore considered to beapplicable only to some limited materials and production methods.

On the other hand, when a nickel-based heat-resistant superalloyproduced by a casting and forging method is subjected to solution heattreatment in a temperature region exceeding a solvus temperature,crystal grains become too large and therefore heat-resistantcharacteristics are significantly impaired. Therefore, in general,solution heat treatment is performed at 90% or less of a solvustemperature, and then aging heat treatment is performed.

At present, however, no nickel-based heat-resistant superalloy has beenfound which is produced by a conventional casting and forging method andhas heat-resistant characteristics significantly higher than those ofnickel-based heat-resistant superalloys produced by powder metallurgy.Therefore, there is a strong demand for development of a nickel-basedheat-resistant superalloy that is produced by a casting and forgingmethod capable of significantly simplifying its production process andthat is superior also in terms of heat-resistant characteristics andcost to nickel-based heat-resistant superalloys produced by powdermetallurgy.

Patent Literature 1: WO 2006/059805

Patent Literature 2: Japanese Patent No. 2666911

Patent Literature 3: Japanese Patent No. 2667929

Patent Literature 4: JP 2003-89836 A

Patent Literature 5: U.S. Pat. No. 4,957,567

Patent Literature 6: U.S. Pat. No. 5,529,643

Patent Literature 7: JP 2011-12346 A

Patent Literature 8: JP 2009-7672 A

SUMMARY OF INVENTION Technical Problem

In order to achieve an improvement in energy efficiency, there hasrecently been an urgent need for development of a material ofheat-resistant members of aircraft engines, power-generating gasturbines, etc. to allow the heat-resistant members to be used at highertemperatures. For example, there has been a strong demand fordevelopment of a novel alloy for turbine disks which is superior inmechanical characteristics such as fatigue strength, high-temperaturecreep strength, fracture toughness, and high-temperature fatigue crackresistance.

Under circumstances where no nickel-based heat-resistant superalloy hasbeen found which is produced by a conventional casting and forgingmethod and has heat-resistant characteristics significantly higher thanthose of nickel-based heat-resistant superalloys produced by powdermetallurgy, the present inventors have made an intensive study todevelop a nickel-based heat-resistant superalloy that is superior interms of heat-resistant characteristics and cost to those produced bypowder metallurgy. It is an object of the present invention to provide anickel-based heat-resistant superalloy that is produced by a casting andforging method capable of significantly simplifying its productionprocess and that is superior in heat-resistant characteristics tonickel-based superalloys produced by powder metallurgy.

Solution to Problem

The present inventors have intensively studied the solution heattreatment conditions of a nickel-based heat-resistant superalloyproduced by a casting and forging method and having a specific alloycomposition, and have found that a nickel-based heat-resistantsuperalloy excellent in both tensile strength and creep life at hightemperature can be obtained by properly controlling especially asolution heat treatment temperature, which has led to the completion ofthe present invention. A casting and forging method is generally knownas an inexpensive production process, and the present inventors havefound that a nickel-based heat-resistant superalloy superior inhigh-temperature heat-resistant characteristics, which can be achievedonly by powder metallurgy requiring high production cost, can beproduced by a casting and forging method.

More specifically, the present invention is directed to a nickel-basedheat-resistant superalloy produced by a casting and forging method, thenickel-based heat-resistant superalloy comprising 2.0 mass % or more but25 mass % or less of chromium, 0.2 mass % or more but 7.0 mass % or lessof aluminum, 19.5 mass % or more but 55.0 mass % or less of cobalt,[0.17×(mass % of cobalt content−23)+3] mass % or more but [0.17×(mass %of cobalt content−20)+7] mass % or less and 5.1 mass % or more oftitanium, and the balance being nickel and inevitable impurities, andbeing subjected to solution heat treatment at 93% or more but less than100% of a γ′ solvus temperature.

It is preferred that in the nickel-based heat-resistant superalloy, thecobalt is contained in an amount of 21.8 mass % or more but 55.0 mass %or less.

Further, it is also preferred that in the nickel-based heat-resistantsuperalloy, the titanium is contained in an amount of 5.5 mass % or morebut 12.44 mass % or less.

Further, it is also preferred that in the nickel-based heat-resistantsuperalloy, the titanium is contained in an amount of 6.1 mass % or morebut 12.44 mass % or less.

Further, it is also preferred that the nickel-based heat-resistantsuperalloy is subjected to solution heat treatment at 94% or more butless than 100% of the γ′ solvus temperature.

Further, it is also preferred that the nickel-based heat-resistantsuperalloy contains one or both of 10 mass % or less of molybdenum and10 mass % or less of tungsten.

Further, it is also preferred that in the nickel-based heat-resistantsuperalloy, the molybdenum is contained in an amount of less than 4 mass%.

Further, it is also preferred that in the nickel-based heat-resistantsuperalloy, the tungsten is contained in an amount of less than 3 mass%.

Further, it is also preferred that the nickel-based heat-resistantsuperalloy contains one or both of 10 mass % or less of tantalum and 5.0mass % or less of niobium.

Further, it is also preferred that the nickel-based heat-resistantsuperalloy contains at least one of 2 mass % or less of vanadium, 5 mass% or less of rhenium, 0.1 mass % or less of magnesium, 2 mass % or lessof hafnium, and 3 mass % or less of ruthenium.

Further, it is also preferred that the nickel-based heat-resistantsuperalloy comprises 12 mass % or more but 14.9 mass % or less ofchromium, 2.0 mass % or more but 3.0 mass % or less of aluminum, 20.0mass % or more but 27.0 mass % or less of cobalt, 5.5 mass % or more but6.5 mass % or less of titanium, 0.8 mass % or more but 1.5 mass % orless of tungsten, 2.5 mass % or more but 3.0 mass % or less ofmolybdenum, at least one of 0.01 mass % or more but 0.2 mass % or lessof zirconium, 0.01 mass % or more but 0.15 mass % or less of carbon, and0.005 mass % or more but 0.1 mass % or less of boron, and the balancebeing nickel and inevitable impurities.

The nickel-based heat-resistant superalloy according to the presentinvention that satisfies the following three requirements is excellentin both tensile strength and creep life at high temperature:

1) being a nickel-based heat-resistant superalloy produced by a castingand forging method;

2) comprising 2.0 mass % or more but 25 mass % or less of chromium, 0.2mass % or more but 7.0 mass % or less of aluminum, 19.5 mass % or morebut 55.0 mass % or less of cobalt, [0.17×(mass % of cobaltcontent−23)+3] mass % or more but [0.17×(mass % of cobalt content−20)+7]mass % or less and 5.1 mass % or more of titanium, and the balance beingnickel and inevitable impurities; and

3) be subjected to solution heat treatment in a temperature region of93% or more but less than 100% of a γ′ solvus temperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a relationship between creep life (hr) and the ratio ofsolution heat treatment temperature (T) to γ′ solvus temperature (Ts),which was determined by a creep test performed under conditions of 725°C. and 630 MPa.

FIG. 2 shows a comparison of creep life among Inventive alloys 1 to 3and Reference alloy 1 (test temperature: 725° C., applied stress: 630MPa) when the ratio of solution heat treatment temperature (T) to γ′solvus temperature (Ts) was set to a constant value of 99%.

FIG. 3 shows a relationship between 0.2% proof stress (test temperature:750° C.) and creep life (test temperature: 725° C., applied stress: 630MPa) of Inventive alloys 1 to 3 and Reference alloys 1 to 5.

DESCRIPTION OF EMBODIMENTS

As described above, when a nickel-based heat-resistant superalloyproduced by a casting and forging method is subjected to solution heattreatment in a temperature region exceeding a solvus temperature,crystal grains generally become too large and therefore heat-resistantcharacteristics are significantly impaired. Particularly, it is saidthat tensile strength (0.2% proof stress) is significantly reduced.Further, it is said that even when the solution heat treatment isperformed in a temperature region equal to or less than a solvustemperature (at a subsolvus temperature), crystal grains become coarsewith an increase in solution heat treatment temperature, and thereforetensile strength (0.2% proof stress) is significantly reduced (e.g., J.C. Williams et al. Acta Mater, 51 (2003) 5775). However, the presentinventors have found that even when produced by a casting and forgingmethod, a nickel-based heat-resistant superalloy that is subjected tosolution heat treatment not at a solution heat treatment temperaturecommonly used but at a high temperature of 93% or more but less than100% of a γ′ solvus temperature is excellent in both tensile strength(0.2% proof stress) and creep life even in a temperature region, inwhich excellent tensile strength and excellent creep life cannotconventionally be achieved, as long as the nickel-based heat-treatmentsuperalloy is a high-cobalt and high-titanium alloy containing 19.5 mass% or more but 55.0 mass % or less of cobalt and [0.17×(mass % of cobaltcontent−23)+3] mass % or more but [0.17×(mass % of cobalt content−20)+7] mass % or less and 5.1 mass % or more of titanium.

A nickel-based heat-resistant superalloy according to the presentinvention contains, as major constituent elements, chromium, cobalt,titanium, aluminum, and nickel and may contain an addition ingredientand an inevitable impurity element.

Chromium is added to improve environment resistance or fatigue crackpropagation characteristics. If a chromium content is less than 1.0 mass%, a desired improvement in these characteristics cannot be achieved,and if the chromium content exceeds 30.0 mass %, a harmful TCP phase islikely to be formed. Therefore, the chromium content is 2.0 mass % ormore but 25.0 mass % or less, preferably 5.0 mass % or more but 20.0mass % or less, more preferably 12 mass % or more but 14.9 mass % orless.

Cobalt is a component useful for controlling a ₇′ phase solvustemperature. An increase in cobalt content reduces the γ′ solvustemperature and widens a process window (ranges of various conditions inwhich a process such as forging can be industrially performed), andtherefore a forgeability-improving effect can also be obtained.Particularly, when titanium is contained in a large amount, cobalt canbe added in a slightly larger amount to suppress a TCP phase and improvehigh-temperature strength. The cobalt content is usually 19.5 mass % ormore but 55.0 mass % or less. Based on the result of a high-temperaturecompression test, the compressive strength of a nickel-basedheat-resistant superalloy whose cobalt content exceeds 55.0 mass % tendsto reduce in a temperature region from room temperature to 750° C.Therefore, the upper limit of the cobalt content is generally 55.0 mass%. The cobalt content is more preferably 19.5 mass % or more but 35.0mass % or less, even more preferably 21.8 mass % or more but 27.0 mass %or less.

Titanium is an addition element preferably used to strengthen a γ′ phaseto improve strength. A titanium content is usually 2.5 mass % or morebut 15.0 mass % or less. When titanium is added in combination withcobalt, a more beneficial effect can be obtained by adding 5.1 mass % ormore but 15.0 mass % or less of titanium. The addition of titanium incombination with cobalt makes it possible to achieve a nickel-basedheat-resistant superalloy having excellent phase stability and highstrength. Basically, a nickel-based heat-resistant superalloy that isstable in structure and has high strength even at a high alloyconcentration can be achieved by selecting a heat-resistant superalloyhaving a γ/γ′ two-phase structure and adding a Co—Co₃Ti alloy having aγ/γ′ two-phase structure just like the heat-resistant superalloy. Inthis case, the titanium content is within a range represented by thefollowing formula.

That is, the titanium content is 0.17×(mass % of cobalt−23)+3 or morebut 0.17×(mass % of cobalt−20)+7 or less.

However, if the titanium content exceeds 15.0 mass %, it is often thecase that the formation of an phase that is a harmful phase becomesconspicuous. Therefore, the upper limit of the titanium content ispreferably 12.44 mass %. The titanium content is more preferably 5.5mass % or more but 12.44 mass % or less, even more preferably 6.1 mass %or more but 11.0 mass % or less.

Aluminum is an element that forms a γ′ phase, and an aluminum content isadjusted to form a γ′ phase in a proper amount. The aluminum content is0.2 mass % or more but 7.0 mass % or less. Further, the ratio betweenthe titanium content and the aluminum content is strongly linked to theformation of an phase, and therefore in order to suppress the formationof a TCP phase that is a harmful phase, the aluminum content ispreferably high to some extent. Further, aluminum is directly involvedin the formation of an aluminum oxide on the surface of a nickel-basedheat-resistant superalloy and is also involved in oxidation resistance.The aluminum content is preferably 1.0 mass % or more but 6.0 mass % orless, more preferably 2.0 mass % or more but 3.0 mass % or less.

Further, the nickel-based heat-resistant superalloy according to thepresent invention may contain the following elements as additioningredients.

Molybdenum mainly has the effect of strengthening a γ phase andimproving creep characteristics. Molybdenum is a high-density element,and therefore if its content is too high, the density of a nickel-basedheat-resistant superalloy is increased, which is not preferred from apractical viewpoint. The molybdenum content is usually 10 mass % orless, preferably less than 4 mass %, more preferably 2.5 mass % or morebut 3.0 mass % or less.

Tungsten is an element that is dissolved in a γ phase and a γ′ phase andstrengthens both the phases, and is therefore effective at improvinghigh-temperature strength. If a tungsten content is low, there is a casewhere creep characteristics are poor. On the other hand, if the tungstencontent is high, there is a case where the density of a nickel-basedheat-resistant superalloy is increased because tungsten is ahigh-density element just like molybdenum. The tungsten content isusually 10 mass % or less, preferably less than 3 mass %, 0.8 mass % ormore but 1.5 mass % or less.

Tantalum is effective as a strengthening element. On the other hand, ifa tantalum content is high to some extent, a nickel-based heat-resistantsuperalloy has a high specific gravity and becomes expensive. Thetantalum content is usually preferably 10 mass % or less.

Niobium is effective as a strengthening element and is also effective atcontrolling a specific gravity. On the other hand, if its content ishigh to some extent, there is a possibility that an undesirable phase isformed or cracks occur during hardening at high temperature. The niobiumcontent is usually 5.0 mass % or less, preferably 0.1 mass % or more but4.0 mass % or less.

The nickel-based heat-resistant superalloy according to the presentinvention may also contain, as another element, at least one elementselected from vanadium, rhenium, magnesium, hafnium, and ruthenium aslong as its characteristics are not impaired. For example, a vanadiumcontent is 2 mass % or less, a rhenium content is 5 mass % or less, amagnesium content is 0.1 mass % or less, a hafnium content is 2 mass %or less, and a ruthenium content is 3 mass % or less. Ruthenium iseffective at improving heat resistance and workability.

Further, the nickel-based heat-resistant superalloy according to thepresent invention may contain, as another element, at least one elementselected from zirconium, carbon, and boron as long as itscharacteristics are not impaired. Zirconium is an element effective atimproving ductility, fatigue characteristics, etc. Usually, a zirconiumcontent is preferably 0.01 mass % or more but 0.2 mass % or less.

Carbon is an element effective at improving ductility and creepcharacteristics at high temperature. Usually, a carbon content is 0.01mass % or more but 0.15 mass % or less, preferably 0.01 mass % or morebut 0.10 mass % or less, more preferably 0.01 mass % or more but 0.05mass % or less. Boron can improve creep characteristics, fatiguecharacteristics, etc. at high temperature. Usually, a boron content is0.005 mass % or more but 0.1 mass % or less, preferably 0.005 mass % ormore but 0.05 mass % or less, more preferably 0.01 mass % or more but0.03 mass % or less. If the carbon content and boron content exceedtheir respective ranges described above, there is a case where creepstrength is reduced or a process window becomes narrow.

The nickel-based heat-resistant superalloy according to the presentinvention is produced by melting a blended raw material having theabove-described composition to prepare an ingot and forging this ingot.The nickel-based heat-resistant superalloy according to the presentinvention having a high cobalt content and a high titanium content has awide process window and excellent forgeability and therefore can beproduced efficiently. The prepared forged material is subjected tosolution heat treatment and then to aging heat treatment so that thenickel-based heat-resistant superalloy according to the presentinvention is obtained. The nickel-based heat-resistant superalloyaccording to the present invention having a high cobalt content and ahigh titanium content and treated in the process of solution heattreatment in a high temperature region of 93% or more but less than100%, preferably 94% or more but less than 100% of a γ′ solvustemperature is excellent in both tensile strength and creep life even ina high temperature region in which excellent tensile strength andexcellent creep life cannot conventionally be achieved.

A nickel-based heat-resistant superalloy is generally forged at a solvustemperature or higher at which the nickel-based heat-resistantsuperalloy has a single phase, because if a γ′ phase that is aprecipitation strengthening phase is present, ductility is reduced. Onthe other hand, the nickel-based heat resistant superalloy according tothe present invention having a high cobalt content and a high titaniumcontent exhibits excellent forgeability even in a temperature regionless than a γ′ solvus temperature. Therefore, the nickel-basedheat-resistant superalloy according to the present invention forged insuch a temperature region is excellent in both creep life and tensilestrength and is very suitable for practical use.

Hereinbelow, the nickel-based heat-resistant superalloy according to thepresent invention will be described in more detail with reference toexamples. As a matter of course, the present invention is not limited tothe following examples.

EXAMPLES

Ingots of three kinds of inventive alloys (Inventive alloys 1 to 3)having compositions shown in Table 1 were prepared by triple melting inwhich three different melting processes, that is, vacuum inductionmelting, electroslag remelting, and vacuum arc remelting were performed,and were then subjected to homogenization heat treatment at about 1200°C. Then, the ingots were forged at 1100° C. on average to producesimulated turbine disks. Further, as comparative samples, simulatedturbine disks were produced using typical existing alloys (Referencealloys 1 to 5) in the same manner as described above. The chemicalcompositions of the reference alloys are also shown in Table 1.

TABLE 1 γ′ solvus Alloy composition (mass %) temperature Alloy number NiCr Mo W Co Ti Al (° C.) Notes Inventive alloy 1 Balance 13.5 2.8 1.225.0 6.2 2.3 ≈1162 TMW alloy Inventive alloy 2 Balance 13.8 2.6 1.1 25.05.6 2.2 ≈1150 TMW alloy Inventive alloy 3 Balance 14.4 2.7 1.1 21.8 6.22.3 ≈1166 TMW alloy Reference alloy 1 Balance 16.0 3.0 1.25 15.0 5.0 2.5≈1158 U720Li Reference alloy 2 Balance 15.0 5.0 — 19.0 3.3 4.3 —Udimet700 Reference alloy 3 Balance 18.0 4.0 — 18.0 3.0 2.9 — Udimet500Reference alloy 4 Balance 19.0 10.0 — 11.0 3.2 1.5 — Rene41 Referencealloy 5 Balance 19.5 4.25 — 13.5 3.0 1.3 — Waspaloy

The simulated turbine disks obtained by casting and forging Inventivealloys 1 to 3 and Reference alloy 1 (U720Li) were subjected to heattreatment in air for 4 hours at different solution heat treatmenttemperatures and then subjected to aging heat treatment. After thetreatment, the samples were subjected to a creep life test. FIG. 1 showsa relationship between the ratio of solution heat treatment temperature(T) to γ′ solvus temperature (Ts) (T/Ts) and creep life. As can be seenfrom FIG. 1, the creep life was excellent when the ratio of solutionheat treatment temperature (T) to γ′ solvus temperature (Ts) (T/Ts) wasset to about 0.93 or more but less than 1.0. When the solution heattreatment temperature (T) was equal to or higher than the γ′ solvustemperature (Ts), the creep life was rapidly reduced. Further, in thecase of Reference alloy 1 (U720Li) showing the best performance amongthe existing nickel-based heat-resistant superalloys, a significantimprovement in creep life was not observed even when the ratio ofsolution heat treatment temperature (T) to γ′ solvus temperature (Ts)was brought close to 1.0, and its creep life was shorter than those ofInventive alloys 1 to 3. It has been found from these results that thenickel-based heat-resistant superalloys according to the presentinvention produced by a casting and forging method and having a highcobalt content and a high titanium content specifically exhibitexcellent creep life when the ratio of solution heat treatmenttemperature (T) to γ′ solvus temperature (Ts) (T/Ts) is set to about0.93 or more but less than 1.0.

FIG. 2 shows a comparison of creep life among Inventive alloys 1 to 3and Reference alloy 1 when the ratio of solution heat treatmenttemperature (T) to γ′ solvus temperature (Ts) was a constant value of99% (test temperature: 725° C., applied stress: 630 MPa). As can be seenfrom FIG. 2, the nickel-based heat-resistant superalloys according tothe present invention having a high cobalt content and a high titaniumcontent have a creep life about three to five times that of thecommercially-available reference alloy (U720Li).

FIG. 3 shows a relationship between 0.2% proof stress (test temperature:750° C.) and creep life (test temperature: 725° C., applied stress: 630MPa) of Inventive alloys 1 to 3 and Reference alloys 1 to 5. As can beseen from FIG. 3, the nickel-based heat-resistant superalloys accordingto the present invention have not only significantly-improved creep lifeas compared to the existing nickel-based heat-resistant superalloys butalso excellent tensile strength.

The above test results demonstrate that a nickel-based heat-resistantsuperalloy that satisfies the following three requirements is excellentin both creep life and tensile strength and is very suitable forpractical use:

1) being a nickel-based heat-resistant superalloy produced by a castingand forging method;

2) comprising 2.0 mass % or more but 25 mass % or less of chromium, 0.2mass % or more but 7.0 mass % or less of aluminum, 19.5 mass % or morebut 55.0 mass % or less of cobalt, [0.17×(mass % of cobaltcontent−23)+3] mass % or more but [0.17×(mass % of cobalt content−20)+7]mass % or less and 5.1 mass % or more of titanium, and the balance beingnickel and inevitable impurities; and

3) be subjected to solution heat treatment at 93% or more but less than100% of a γ′ solvus temperature.

INDUSTRIAL APPLICABILITY

It is possible to provide a nickel-based heat-resistant superalloymainly having significantly-improved heat-resistant characteristics. Thenickel-based heat resistant superalloy is useful for heat-resistantmembers of aircraft engines, power-generating gas turbines, etc.,especially for high-temperature·high-pressure turbine disks, compressorblades, shafts, turbine cases, etc.

1. A nickel-based heat-resistant superalloy produced by a casting andforging method, the nickel-based heat-resistant superalloy comprising2.0 mass % or more but 25.0 mass % or less of chromium, 0.2 mass % ormore but 7.0 mass % of less of aluminum, 19.5 mass % or more but 55.0mass % or less of cobalt, [0.17×(mass % of cobalt content−23)+3] mass %or more but [0.17×(mass % of cobalt content−20)+7] mass % or less and5.1 mass % or more of titanium, and the balance being nickel andinevitable impurities, and being subjected to solution heat treatment at93% or more but less than 100% of a γ′ solvus temperature.
 2. Thenickel-based heat-resistant superalloy according to claim 1, wherein thecobalt is contained in an amount of 21.8 mass % or more but 55.0 mass %or less.
 3. The nickel-based heat-resistant superalloy according toclaim 1, wherein the titanium is contained in an amount of 5.5 mass % ormore but 12.44 mass % or less.
 4. The nickel-based heat-resistantsuperalloy according to claim 3, wherein the titanium is contained in anamount of 6.1 mass % or more but 12.44 mass % or less.
 5. Thenickel-based heat-resistant superalloy according to claim 1, which issubjected to solution heat treatment at 94% or more but less than 100%of the γ′ solvus temperature.
 6. The nickel-based heat-resistantsuperalloy according to claim 1, which contains one or both of 10 mass %or less of molybdenum and 10 mass % or less of tungsten.
 7. Thenickel-based heat-resistant superalloy according to claim 6, wherein themolybdenum is contained in an amount of less than 4 mass %.
 8. Thenickel-based heat-resistant superalloy according to claim 6, wherein thetungsten is contained in an amount of less than 3 mass %.
 9. Thenickel-based heat-resistant superalloy according to claim 1, whichcontains one or both of 10 mass % or less of tantalum and 5.0 mass % orless of niobium.
 10. The nickel-based heat-resistant superalloyaccording to claim 1, which contains at least one of 2 mass % or less ofvanadium, 5 mass % or less of rhenium, 0.1 mass % or less of magnesium,2 mass % or less of hafnium, and 3 mass % or less of ruthenium.
 11. Thenickel-based heat-resistant superalloy according to claim 1, whichcomprises 12 mass % or more but 14.9 mass % or less of chromium, 2.0mass % or more but 3.0 mass % or less of aluminum, 20.0 mass % or morebut 27.0 mass % or less of cobalt, 5.5 mass % or more but 6.5 mass % orless of titanium, 0.8 mass % or more but 1.5 mass % or less of tungsten,2.5 mass % or more but 3.0 mass % or less of molybdenum, at least one of0.01 mass % or more but 0.2 mass % or less of zirconium, 0.01 mass % ormore but 0.15 mass % or less of carbon, and 0.005 mass % or more but 0.1mass % or less of boron, and the balance being nickel and inevitableimpurities.